Referring to FIG. 8, there is shown an explanatory view showing an operating principle of a conventional redox flow secondary battery. The redox flow battery has a cell 1 separated into a positive electrode cell 1A and a negative electrode cell 1B by a membrane 4 that can allow ions to pass through. The positive electrode cell 1A and the negative electrode cell 1B include a positive electrode 5 and a negative electrode 6, respectively. A positive electrode tank 2 for feeding and discharging positive electrolytic solution to and from the positive electrode cell 1A is connected to the positive electrode cell 1A through conduit pipes 7, 8. Similarly, a negative electrode tank 3 for feeding and discharging negative electrolytic solution to and from the negative electrode cell 1B is connected to the negative electrode cell 1B through conduit pipes 10, 11. Aqueous solution containing ions that change in valence, such as vanadium ion, is used for the positive and negative electrolytes. The electrolyte containing the ions is circulated by using pumps 9, 12, to charge and discharge with the change in ionic valence at the positive electrodes 5 and negative electrodes 6.
Referring to FIG. 9, there is shown a diagrammatic illustration of construction of a cell stack used for the redox flow battery mentioned above. This type of battery usually uses the construction which is called a cell stack 100 comprising a plurality of cell frames 20 stacked in layers.
The cell stack 100 comprises a stack body formed by a cell frame 20, a positive electrode 5 made of carbon felt, a membrane 4, a negative electrode 6 made of carbon felt and the cell frame 20 being repeatedly stacked in this sequence. End plates are arranged at both sides of the stack body and are clamped onto the both sides of the stack body by tightening nuts screwably engaged with long bolts 101 piercing the both end plates, to thereby produce the cell stack 100.
The cell frame 20 comprises a bipolar plate 21 made of plastic carbon and a frame 22 formed around a periphery of the bipolar plate. The cell frame 22 usually has, in lower and upper sides thereof, holes which are called manifolds 23A, 23B for feeding and discharging the electrolytes to and from their respective cells and guide grooves 24 extending continuously from the manifolds for guiding the electrolyte to the electrodes 5, 6.
Referring now to FIG. 10, there is shown a partially enlarged view schematically showing a section around a frame when conventional cell frames are stacked in layer. A seal using an O-ring (FIG. 10(a)-(c)) disclosed by Japanese Laid-open (Unexamined) Patent Publication No. 2000-260460 and a seal using a flat packing (FIG. 10(d)) disclosed by Japanese Laid-open (Unexamined) Patent Publication No. Hei 8-7913 are known as a mechanism for preventing leakage of electrolyte from between the cell frames.
Cell frames 20a shown in FIG. 10(a) each have O-ring grooves 25 formed at locations opposite to each other on both sides thereof, one for each side, and O-rings 26 are fitted in the O-ring grooves 25.
Cell frames 20b shown in FIG. 10(b) each have an inner O-ring groove 25a formed on one side thereof and an outer O-ring groove 25b formed on the other side, both grooves being provided at locations staggered with respect to each other, and an inner O-ring 26a and an outer O-ring 26b are fitted in the grooves 25a, 25b, respectively.
Cell frames 20c shown in FIG. 10(c) each have the inner O-ring groove 25a and the outer O-ring groove 25b, different in size from each other, which are formed on one side thereof, so that one pair of the grooves 25a, 25b is arranged in parallel with each other, and the inner O-ring 26a and the outer O-ring 26b are fitted in the grooves 25a, 25b, respectively, as is the case with the above.
Cell frames 20d shown in FIG. 10(d) each have a flat packing 27, corresponding in shape to the cell frame 20d, which is arranged on each side.
For a redox flow battery of a relatively small size, a seal using a heat fusion bonding method listed in “provisions for power storage battery system” is also known.
The cell stacks using the conventional cell frames described above have the following problems, however.
(1) It is difficult to prevent leakage of electrolyte from between the cell frames effectively,
{circle around (1)} In the cell stack using the cell frames 20a-20c shown in FIG. 10(a)-(c), part of the membrane 4 projected outwardly of the O-ring 26, 26a is not kept in its wet condition due to dryness and thus is sometimes broken. When the break in the membrane progresses inwardly with respect to the O-ring 26, 26a, there is a possibility that the electrolyte may leak out of the cell frames 20a-20c through that break.
{circle around (2)} The flat packing 27 shown in FIG. 10(d) is desirable for the cell frame of a large area to produce a high capacity. However, when the cell frames 20d are stacked in layers, with the flat packing 27 interposed therebetween, the flat packing 27 must be positioned precisely with respect to the cell frames and also the cell frames 20d stacked in layers must be clamped uniformly by a number of long bolts 101, in order to prevent the leakage of electrolyte.
(2) Workability in a Cell Stack Assembly is Poor
{circle around (1)} In the cell stack using the cell frames 20b, 20c shown in FIGS. 10(b) and (c), since the membrane 4 between the cell frames is set so that its periphery is positioned to be above the inner O-ring 26a but inside of the outer O-ring 26b, the membrane 4 must be cut to extremely close tolerance. In addition, the membrane 4 cut with a very high degree of precision must be aligned to the cell frames precisely, thus involving very poor workability in producing the cell stack.
{circle around (2)} In the cell stack using the flat packing 27 shown in FIG. 10(d), the flat packing 27 must be also aligned to the cell frames 20d precisely, thus involving very poor workability in assembling the cell stack.
{circle around (3)} In the seal using a heat fusion bonding method, as a size of the cell frame increases, the fusion bonding work becomes complicated, involving difficulties in the application of the seal. Also, the use of this type of seal causes cost increase.
Accordingly, it is a primary object of the present invention to provide a cell frame for a redox flow battery that effectively prevents leakage of electrolyte out of the cell frame and also provides a good workability in assembling the redox flow battery.
It is another object of the present invention to provide a redox flow battery using that cell frame.